A digital subscriber line (DSL) technology is a broadband access technology for transmitting a high-speed data service based on a transmission medium of an ordinary telephone twisted pair (unshielded twist pair, UTP). The digital subscriber line technology mainly uses a frequency division multiplexing technology to implement coexistence of a digital subscriber line service and a POTS plain old telephone service) on a same ordinary telephone twisted pair, without replacing an existing basic transmission medium, and can obtain a high downstream/upstream transmission speed when data transmission is performed by using the existing ordinary telephone twisted pair. In an entire transmission frequency band, the POTS service occupies a baseband part lower than 4 kHz, while a digital subscriber line service occupies a high frequency part, where signals of the two service are separated by using a splitter. The digital subscriber line technology mainly uses a modulation mode, that is, DMT (discrete multi-tone), to improve an anti-interference capability of a digital subscriber line system.
Referring to FIG. 1, FIG. a in FIG. 1 is a schematic diagram of near end crosstalk, and FIG. b is a schematic diagram of far end crosstalk. In a digital subscriber line system, a DSLAM (digital subscriber line access multiplexer, Digital Subscriber Line Access Multiplexer) 101, as a central office end device in the digital subscriber line system, can accommodate multiple DSL lines and optimize a transmission speed. By using two DSL lines, that is, a first line 103 established between a central office end transceiver 1011 and a subscriber end 1021 and a second line 104 established between a central office end transceiver 1012 and a subscriber end 1022, as an example, according to an electromagnetic induction principle, crosstalk is generated between signals of the first line 103 and the second line 104 accessed to the DSLAM. Crosstalk is classified into FEXT (far end crosstalk) and NEXT (near end crosstalk). FEXT refers to interference between upstream signals of different line pairs or between downstream signals of different line pairs, while NEXT is interference between upstream signals and downstream signals of different line pairs.
Both FEXT and NEXT become stronger as the frequency band is higher; and a frequency division multiplexing mode is used in a downstream/upstream channel of a digital subscriber line system. Therefore, NEXT may be canceled or reduced by using a filter, and no great impact is caused to the digital subscriber line system. However, as the frequency band used by the digital subscriber line system is wider, FEXT also becomes stronger continuously. According to Shannon's equation C=B·log2(1+SiN) (where C is a channel speed, B is a signal bandwidth, S is signal energy, and N is noise energy), it may be known that if N is larger, C is smaller. In digital subscriber line transmission, crosstalk is reflected as a part of noise, and stronger FEXT indicates larger N. Therefore, severe FEXT dramatically reduces the channel speed. Therefore, when multiple subscribers request to activate a digital subscriber line service in a bundle of cables, because of FEXT, transmission speeds of a part of lines are low, and performance is instable, and even the activation fails, finally resulting in a low service activation rate of the DSLAM.
In view of the foregoing problem, currently, a vectoring technology is proposed in the industry. Mainly, in a DSLAM, downstream precoding and upstream joint reception technologies are used to implement crosstalk cancellation, and by means of interaction between the DSLAM and a terminal, crosstalk vector information in a line is acquired, and then complex matrix calculation is performed to acquire an “inverse” crosstalk signal, and the “inverse” crosstalk signal is superimposed into a digital subscriber line signal. In a transmission process of the digital subscriber line signal, the “inverse” crosstalk signal and FEXT in the line cancel each other, and therefore, impact of the FEXT on line transmission performance is reduced.
In a long-term research and development process, the inventors of the present invention find that the following technical problem exists in the use of the foregoing vectoring technology: At an initial stage of line initialization, a nonlinear precoding technology cannot be used for signal precoding, but using linear coding easily causes a dramatic increase of transmit power. By using crosstalk cancellation in a downstream channel as an example, a vector precoder is introduced in a CO (central office, Central office), and in joint transmission of signals, after a signal is precoded by the vector precoder, an “inverse” crosstalk signal is acquired, and FEXT of the signal is canceled in a transmission process by using the “inverse” crosstalk signal. In the use of the vectoring technology, a crosstalk channel and/or a crosstalk cancellation coefficient needs to be acquired for signal precoding. Precoding modes mainly include linear precoding and nonlinear precoding. When a signal is sent by using a low frequency band, FEXT can be well canceled by using the linear precoding mode. However, as the frequency band becomes higher, when a new line is added, a linear precoder causes an apparent increase of transmit power of a Showtime (transmitting line) port. However, in ordinary devices and systems, transmit power of all ports has an upper limit. Therefore, using the linear precoding mode may cause the transmit power of the Showtime port to exceed the limit. To avoid that the transmit power exceeds the limit when crosstalk cancellation is completed, the nonlinear precoding technology may be used for signal precoding. However, before this mode is used, it is necessary to know all crosstalk channels. At an initial stage of initialization of a new line, before a feedback channel is established for the initializing line, the initializing line can hardly feed back an error signal of a downstream channel through an upstream channel, and a crosstalk channel from a Showtime line to the initializing line and a crosstalk channel from the initializing line to another initializing line cannot be known. Therefore, at the initial stage of line initialization, the nonlinear precoding technology cannot be used to perform signal precoding to cancel crosstalk; and using linear precoding easily causes a dramatic increase of transmit power.